Titanium dioxide (TiO2) is commonly viewed as being the principal white pigment in commerce. It has an exceptionally high refractive index, negligible colour and is also inert. Titanium dioxide is generally present in the market place in either of two predominant polymorphs, anatase or rutile; for the majority of commercial applications, rutile is the desired form. Titanium dioxide is well known as being useful as an opacifier in paints, paper, plastics, ceramic, inks, etc. Titanium dioxide, as sold commercially, generally has an average particle size of 150 nm to 350 nm.
There are two main processes for making raw pigmentary titanium dioxide: the sulfate process and the chloride process.
The sulfate process is based on the digestion of ilmenite or titania slag in concentrated sulfuric acid. After iron removal as iron sulfate, the solution is heated and diluted with water. The titanium hydrolyzes, forming a titanium oxysulfate precipitate, which is further treated to produce TiO2 pigment.
The chloride process relies on the carbo-chlorination of low-iron, titanium containing ore or intermediate products to form TiCl4, followed by the gas phase oxidation of TiCl4.
Titanium dioxide can be flocculated and/or precipitated out of a titanium dioxide containing dispersion by pH adjustment of the dispersion.
The finishing process for titanium dioxide, as obtained by any known method, may include one or more of: dry milling, wet milling, classification, filtering, washing, drying, steam micronizing and packaging.
In general, in a commercial process the titanium dioxide dispersion will always be milled and micronized to achieve a desired particle size distribution.
Optionally there may be a surface treatment step. The surface treatment step generally includes precipitating alumina, silica, zirconia, and/or other metal oxides, on the surface of the titanium dioxide. The purpose of this coating treatment is to impart photo stability, shelf life, dispersability, and/or flowability. This step occurs after the wet milling step and before the drying step.
It is generally preferred in the art that the finishing process involves: milling; followed by any required surface treatment step, e.g. metal oxide coating; followed by filtering and/or washing; followed by drying; and then followed by micronizing, to obtain a final titanium dioxide white pigment product having the desired particle size distribution.
The steps of treating and drying the product can cause particles to aggregate and the micronizing step ensures that the particles in the dried and treated product are separated, so that the desired particle size distribution is restored.
Conventionally, the titanium dioxide is always micronized, in order to produce the desired particle size distribution suitable for use as a white pigment in paints, inks or the like where the mechanical energy input during the production of the paint or ink is low.
The particle size distribution in the paint or other pigment-containing product determines the hiding power achieved by the pigment-containing product.
For most paints the mean particle size (when determined using a Brookhaven BI-XDCW X-ray Disc Centrifuge System) should lie in the range of from 0.29 to 0.32 microns, with a geometric standard deviation of less than 1.45. As the skilled person will appreciate, the particle size distribution is modeled as a log normal distribution.
The particle size distribution measurement using a Brookhaven BI-XDCW X-ray Disc Centrifuge System (XDC) may be determined as follows: dried TiO2 material (0.92 g) is mixed with 1/16% sodium silicate solution (16.80 g) and de-ionised water (5.28 g) in a Bosch mill pot to give a dilute suspension of ˜4% solids. The pH is adjusted to between 10 and 10.5 with two drops of sodium hydroxide solution (2%). Samples are then vigorously milled for 10 minutes using a Bosch high-speed impeller. This method is designed to be representative of the mechanical energy used in the production of most paint and inks.
It may also be desired that the particle size distribution does not have a long “tail”, in other words that there is not a significant amount of large size particles present. For example, it is generally desired that 90 wt % or more of the particles should have a particle size that is less than 0.5 microns. A high concentration of particles above 0.5 microns would be detrimental to the gloss of the paint or ink. The particle size diameter can be determined using X-ray sedimentation. Ideally it is also the case that 99 wt % or more of the particles have a particle size diameter (when determined using X-ray sedimentation) that is less than 1.5 microns.
As noted above, the steps of treating and drying the product can cause particles to aggregate, meaning that the fluid energy milling (micronizing) is normally required in a conventional titanium dioxide pigment production route, in order to return the particles to the desired size. Otherwise, when the final product is subsequently dispersed (e.g. by high speed dispersion) in a vehicle, e.g. to form a paint or ink, the resultant product containing particles of titanium dioxide will not have the desired size distribution but instead will have too high a level of oversize particles.
The fluid energy milling is carried out in a fluid-energy mill (or micronizer). Most fluid-energy mills are variations on a basic configuration of a disc-shaped grinding chamber enclosed by two, generally parallel, circular plates defining axial walls, and an annular rim defining a peripheral wall, with the axial length or height of the chamber being substantially less than the diameter. Around the circumference of the mill are located a number of uniformly spaced jets for injecting the grinding fluid, which furnishes additional energy for comminution, along with one or more feed nozzles for feeding the particulate material to be comminuted. The jets are oriented such that the grinding fluid and particulate material are injected tangentially to the circumference of a circle smaller than the chamber circumference. Feed to the grinding chamber can be introduced either through a side inlet that is tangent to the grinding chamber, or at an angle from the top, usually at a 30° angle to the plane of the grinding chamber. Side feed micronizers generally produce the better grinding dispersion, while top feed micronizers can produce higher rates.
Within the grinding chamber, a vortex is formed by the introduction of the grinding fluid such as compressed gas, through the feed inlet or through fluid nozzles positioned in an annular configuration around the periphery of the grinding chamber. The grinding fluid (compressed gas, e.g., air, steam, nitrogen, etc.), fed tangentially into the periphery of the chamber, forms a high-speed vortex as it travels within the grinding chamber. The high-speed vortex sweeps up the particulate material, which results in high speed particle-to-particle collisions as well as collisions with the interior portion of the grinding chamber walls. In the micronizing of titanium dioxide, the grinding fluid is usually superheated steam.
Clearly, heavier particles have longer residence time within the vortex. Lighter particles move with the vortex of gas until the discharge conduit is reached. Typically, fluid-energy mills are capable of producing fine (less than 10 microns diameter) and ultra fine (less than 5 microns diameter) particles. However, during grinding, undesirably large particle sizes can sometimes still be found to escape into the product.
In general, in the white pigment industry, there is a particular need to reduce the amount of oversized material passing prematurely into the resulting pigment product. Thus, the intensity of grinding during micronization is typically increased as compared to when other products are micronized. This has a consequence of higher costs, in terms of fluid use, energy consumption, and reduced capacity per mill.
Further, with such processes, the amount of oversized material may be reduced, but there may be adverse effects on pigment properties.
It is a particular concern that the conventional pigment finishing process is a highly energy intensive process. The highest energy consuming operations in finishing is generally the fluid energy milling of the dried product using superheated steam.
However, this micronizing step cannot simply be omitted. Titanium dioxide pigment produced conventionally but without the use of the fluid energy micronizing step would produce a product that was unsuitable for the production of paints or inks. In particular, the product would not meet the requirements of such products in terms of gloss properties. This is due to the fact that the particle size distribution would be too broad, including a long “tail” of oversize particles.
In U.S. Pat. No. 4,061,503 the treatment of particulate titanium dioxide with a polyether substituted silicon compound is described as a method of enhancing its dispersibility in pigmented and/or filled paints and plastics, and in reinforced plastic composite compositions. The dispersion promoter compound possesses two to three hydrolyzable groups bonded to the silicon and an organic group which contains a polyalkylene oxide group. This compound may be added directly to the plastic, resin or other vehicle containing the titanium dioxide.
U.S. Pat. No. 6,972,301 B2 relates to a process for producing organically modified metal oxides and products thereof. An aqueous dispersion of a metal oxide, which can be peptized in the presence of an acid, is admixed with an aqueous dispersion of an organo silane having the formula RySiX4-y, wherein R is an organic moiety, X is a moiety which produces an acid anion in the presence of water and y is from 1 to 3. The mixture of the aqueous dispersion and the organo silane is then thermally aged to produce a colloidal metal oxide sol.
U.S. Pat. No. 7,381,251 B2 describes mineral particle dispersions stabilized with a poly (oxyalkene) phosphonate. In this regard, a liquid composition is provided that comprises a mixture of: (1) water and/or a polar solvent; (2) a colloidal dispersion of mineral particles; and (3) a phosphonate terminated poly(oxyalkene) polymer.